Figure 5. Schematic representation of coalescence (A) and non-coalescence (B) mechanisms of psW/O droplets within a shift register due to a difference in droplet velocity (solid arrows, velocities above threshold; dashed arrows, velocities below threshold with respect to graph in Figure 2).

During normal device operation, when a droplet reaches an empty shift register it remains trapped within the pillar structure if the correct balance between the hydrodynamic pressure difference between the inlet and outlet points of the register and the Laplace pressure of the droplet interface is achieved. Because the oil is flowing within the register and exiting through the gaps between the pillars, the interface of a trapped droplet can be depleted of phospholipids proportionally to the magnitude of the shear forces parallel to the oil flow directions. When a second droplet, presenting a phospholipid-depleted front interface, enters the register, the oil layer between the two drops is drained through the pillars and contact between the two psW/O droplets will occur. Depending on the phospholipid adsorption dynamics, (i.e. phospholipid concentration and magnitude of hydrodynamic forces), the two droplets will coalesce depending on the degree of phospholipid coverage at both droplet interfaces.